1. Field of the Invention
The present invention relates to a non-aqueous electrolyte for electric cells and a solvent for the electrolyte and particularly it relates to an electrolyte and a solvent for the electrolyte suitable for batteries, i.e., secondary cell or rechargable cell. The present invention further relates to a non-aqueous electrolyte battery utilizing lithium, more specifically, a non-aqueous electrolyte battery comprising a specific non-aqueous solvent where the use of the solvent prevents rapid heat generation and breakdown of the cell possibly caused by overcharge of the cell.
2. Prior Art
Recently, various kinds of portable electronic equipment such as camera-integrated VTR's, portable phones and lap-top computers have been developed and effort to make such equipment smaller and lighter is still being continued. For this reason, batteries have become paid much attention as a portable electric source for the portable electronic equipment and researches for providing batteries capable of realizing high energy density are conducted. In particular, lithium batteries are actively investigated because lithium batteries can provide higher energy density as compared with batteries using aqueous solution electrolyte such as lead batteries and nickel/cadmium cells.
Meanwhile, electric cells utilizing non-aqueous electrolyte have been conventionally and widely used as electric sources of various kinds of consumer electronic equipment because of their high voltage, high energy density and excellent properties relating to reliability of the cells such as storage characteristics and anti-leak property.
Conventional non-aqueous electrolytes usually comprise propylene carbonate and such electrolytes have been widely used for various primary cells including lithium cells because propylene carbonate is a good solvent for supporting salts and stable to alkali metals, and can show excellent discharging characteristics.
While batteries are of course required to show high energy density, excellent discharge characteristics and low self-discharge rate like primary cells, the batteries, in addition to the properties mentioned above, must satisfy the requirements that they show high energy efficiency upon charging and discharging (charge/discharge efficiency) and that the reversibility of the chemical and physical properties of the active materials are maintained upon repetition of charge/discharge cycles. The charge/discharge efficiency and the internal resistance of batteries are greatly influenced by the kind of electrolyte. For example, if electroconductivity of the electrolyte is high, the charge/discharge efficiency is improved and the internal resistance is lowered. Therefore, the electrolytes for batteries are required to have high electroconductivity, anti-redox properties, high withstand voltage and the like.
However, non-aqueous electrolytes generally have electroconductivity tens to hundreds times lower than aqueous solution electrolytes and, in particular, non-aqueous electrolytes having high withstand voltage generally show poor charge/discharge characteristics and insufficient internal resistance. For example, when propylene carbonate is used as the non-aqueous electrolyte for batteries, the charge/discharge efficiency (discharge capacity/charge capacity) is lowered to about 50 to 60%. Further, propylene carbonate cannot provide sufficient electroconductivity because it has high viscosity and hence low ion migration rate.
Although attempts to improve electric double layer conductivity of the cells by adding to propylene carbonate with cyclic ethers such as 1,3-dioxolans and tetrahydrofurans or linear chain ethers such as 1,2-dimethoxyethane (DME) and diethyl ether have been disclosed (e.g., DENKI KAGAKU (Electrochemistry) 53, No. 3, 173 (1985)), the addition of these ether compounds impairs the oxidation resistance of the solvent and then lowers the withstand voltage of the electrolyte.
Further, because of the high reactivity of metal lithium, metal lithium deposited in lithium cells during charge/discharge cycles may react with the solvent. For instance, N-methyl-2-oxazolidinone reacts with metal lithium and hence yellowed. Such reaction may badly affect the charge/discharge efficiency and life time of the cells.
Further, electric cells are generally manufactured in a sealed structure, but when the internal pressure of the cells are increased for some reason the cells may be broken and hence dysfunction. For instance, when non-aqueous electrolyte cells are overcharged by an electric current of larger electricity than expected, electrolyte and so on are decomposed to generate gas and the gas may elevate the internal pressure. Further, if such overcharge state lasts long, abnormal reactions such as rapid decomposition of the electrolyte and the active materials and rapid elevation of the cell temperature may be caused and they may invite breakdown of the cells.
To solve this problem, sealed cells of explosion proof type have been proposed. The cells of this type are provided with a current cut-off device or a pressure release device, which operates in response to the increase of the internal pressure of the cells. For example, if the internal pressure of the cells is elevated as a result of accumulation of gases generated by chemical reactions caused by long lasting overcharge state at inside of the cells, the current cut-off device operates to cut the charging current, or the pressure release device operates to release the internal pressure.
However, if the overcharge state is continued for long time in the conventional sealed cells of explosion proof type, the abnormal exothermic reactions often proceed even after the current cut-off device or the pressure release device has operated, and thus the cell temperature may elevate from 50.degree. or 60.degree. C. to 300.degree. or 400.degree. C. This rapid temperature elevation may cause rapid elevation of the internal pressure, which may lead breakdown of the cells, and is considered as a serious problem.
In view of the above-described problems, the object of the present invention is to provide a novel electrolyte and solvent for the electrolyte excellent in electroconductivity, no(less)-reactivity with alkali metals and withstand voltage. Another object of the present invention is to provide an electrolyte and a solvent for the electrolyte capable of providing batteries with excellent charge/discharge characteristics and stability.
Further, the present invention aims at preventing the abnormal exothermic reactions which may be observed even after the current cut-off device or the pressure release device has operated to prevent the rapid elevation of the cell temperature and the cell internal pressure, thereby breakdown of the cells is avoided.